In a spectrometric measurement system, such as an ultraviolet-visible spectrophotometer or a spectrometric detector for a liquid chromatograph, a photodiode array detector consisting of a large number (e.g. 128 to 1024 pieces) of linearly arrayed photodiodes made of silicon (Si), indium gallium arsenide (InGaAs) or other semiconductor as the base material is used for simultaneous detection of light dispersed into wavelength components by a light-dispersing element (the “photodiode array detector” may also be called a “linear image sensor”, as in Non Patent Literature 1). In the following description, the photodiode array is appropriately abbreviated as PDA.
FIG. 10 is a schematic configuration diagram of a commonly used spectrophotometer using a PDA detector. A measurement light P generated from the emission point of a light source 1 having a predetermined emission spectrum is condensed by a lens 2 and cast into a sample cell 4 made of quartz glass or a similar transparent material in which a solution sample 5 is held. The transmitted light Q which has passed through the solution sample 5 is condensed by a lens 6 and falls onto a light-dispersing element 8, such as a diffraction grating, through a slit 7. The light-dispersing element 8 disperses the transmitted light Q into wavelength components in a one-dimensional direction, and the wavelength-dispersed light S reaches a PDA detector 10. Since the positional relationship between the light-dispersing element 8 and the PDA detector 10 is always the same, each photodiode constituting one picture element on the PDA detector 10 receives a ray of wavelength-dispersed light S with a fixed range of wavelengths at a fixed incident angle. Each photodiode of the PDA detector 10 produces a detection signal corresponding to the intensity (amount) of the incident light. A blank measurement with no incident light can be performed by blocking the measurement light P with a shutter 3 provided between the lens 2 and the sample cell 4. In the present example, the light-dispersing element 8 shown in FIG. 10 is a concave reflection grating, which is capable of forming an image of the slit 7 on the light-receiving surface of the PDA detector 10. In some cases, a lens, mirror or similar optical element having such an image-forming capability may be provided separately from the light-dispersing element 8.
The configuration shown in FIG. 10 is the basic configuration of an absorption spectrophotometer for detecting light transmitted through a sample. In some systems, such as an interferometric film-thickness meter or an emission spectrometer, the solution sample 5 may be replaced by a solid sample or gas sample, or no sample cell 4 holding such samples may be provided at all. The light detected with the PDA detector 10 is not always a ray of light transmitted through a sample; it may be a ray of light reflected or scattered by a sample or directly emitted from the sample. The light-dispersing element 8 is not always a reflection light-dispersing element; it may be a transmission light-dispersing element, such as a prism or a transmission grating. In any of these cases, the spatial arrangement of the components denoted by reference signs 6 through 10 in FIG. 10 is appropriately changed according to the direction of propagation of the transmitted light, reflected (scattered) light, emitted light or other kinds of light to be detected.
In the case of a commonly used ultraviolet-visible spectrophotometer, the required wavelength range is from 200 nm to 1100 nm, i.e. from ultraviolet through near-infrared wavelength regions. To achieve a high level of detection sensitivity in the aforementioned spectrophotometer, each photodiode in the PDA detector needs to have a high level of sensitivity to incident light having a specific wavelength or a wavelength range. For this purpose, it is important to coat the surface of each photodiode with a single-layer or multilayer dielectric coating as an antireflection coating so that incident light can efficiently reach the semiconductor region (photoelectric conversion region) in each photodiode while minimizing the loss of the incident light falling onto the photodiode. For a normally used single photodiode or a PDA consisting of a small number of photodiodes (e.g. a few to several channels), there have been many reports on antireflection coatings capable of allowing incident light to reach the semiconductor region with high efficiency over a limited wavelength range within the ultraviolet through near-infrared wavelength region. However, it is practically impossible to entirely cover the wavelength range from the ultraviolet through near-infrared wavelength region with only a single kind of antireflection coating.
In general, the term “antireflection” in a broad sense includes the function of reducing the surface reflectance by absorbing incident light in the coating layer. However, in the present description, the term “antireflection” is used in a limited sense and means the function of suppressing the reflection of incident light and allowing this light to efficiently reach the semiconductor region.
One example of the detector conventionally used in the previously described type of spectrometric measurement system is a PDA disclosed in Non Patent Literature 1. This type of conventional PDA consists of an array of photodiodes whose surfaces are entirely covered with a surface protection (passivation) coating made of silicon oxide (SiO2), silicon nitride (Si3N4) and/or other semiconductor materials with a uniform thickness. Specifically, the PDA described in Non Patent Literature 1, which is sensitive to a broad range of wavelengths from 190 nm to 1100 nm, has a substrate made of silicon (Si) whose surface is entirely covered with a silicon oxide coating of approximately 1000 nm in thickness. The silicon oxide coating has a refractive index between those of silicon and air, and therefore, shows the effect of increasing the light transmittance by a certain amount. However, this effect of increasing the light transmittance is low within an ultraviolet wavelength region of 380 nm or shorter wavelengths where silicon has a large extinction coefficient (the imaginary part of the refractive index). Furthermore, in this PDA, a wavelength range with high light transmittance and a wavelength range with low light transmittance alternately appear due to the interference effect of the silicon oxide coating, which is reflected in the spectral sensitivity characteristic.
FIG. 11 is one example of the wavelength-transmittance characteristic of an antireflection coating used in a conventional PDA. The antireflection coating is a single-layer SiO2 film with a thickness of 200 nm formed on a silicon substrate. The transmittance characteristic is comparatively flat within a range from visible through near-infrared wavelength regions, with the transmittance maintained at high levels of 60% or higher. By contrast, within a range of 350 nm or shorter wavelengths, there is a wavelength range where the transmittance noticeably decreases, and the fluctuation in the transmittance with respect to the wavelength is considerably large. Within such a wavelength range where the transmittance is extremely low, the detection sensitivity significantly decreases, so that the detection signals produced by those photodiodes which receive light within the aforementioned wavelength range have low signal-to-noise ratios.
The aforementioned large fluctuation in the transmittance due to the interference effect can be suppressed to some extent by using an appropriate multilayer dielectric coating in place of the single-layer SiO2 coating. However, this technique cannot fundamentally solve the problem that the transmittance is extremely low at a portion of the photodiodes and prevents the generation of highly reliable detection signals from those photodiodes.